Our drive to select like-minded partners could be much stronger than previously thought.

Researchers in the US studied a variety of pairs, including those who had just met and long-term couples or friends. 

They found, surprisingly, that those who had known each other longer were not more similar than the newly-met pairs.

This evidence contradicts the idea that couples can change each other over time.

It means similarity is an important factor for striking up a successful relationship from the very beginning, the researchers explained.

'Picture two strangers striking up a conversation on a plane, or a couple on a blind date,' said lead author Angela Bahns, Assistant Professor of Psychology at Wellesley College in Boston.

'From the very first moments of awkward banter, how similar the two people are is immediately and powerfully playing a role in future interactions,' she said.

Whether or not a relationship develops could depend on the level of similarity the two individuals share from the beginning of their meeting.

'You try to create a social world where you're comfortable, where you succeed, where you have people you can trust and with whom you can cooperate to meet your goals,' author Chris Crandall, Professor of Psychology at the University of Kansas said. 

'To create this, similarity is very useful, and people are attracted to it most of the time.'

The study was published today in the Journal of Personality and Social Psychology.

To gather the data, the researchers approached pairs of people interacting in public, whether romantic couples, friends or acquaintances, and asked questions about attitudes, values, prejudices, personality traits and behaviours that were important to them.

The results were compared to see how similar or different the pairs were.

They tested whether pairs who had known each other longer and whose relationships were closer and more intimate were more similar than newly formed pairs. It tuned out they were not.

The researchers also surveyed pairs who had just met, then approached the same pairs again later, allowing them to paint a picture of the same pairs over time.

'Though the idea that partners influence each other is central in relationships research, we have identified a large domain in which friends show very little change-- personality, attitudes and values, and a selection of socially-relevant behaviours,' said Bahns.

And it's not just limited to a couple of particular topics. 'People are more similar than chance on almost everything we measure, and they are especially similar on the things that matter most to them personally,' Bahns said.

'We're arguing that selecting similar others as relationship partners is extremely common so common and so widespread on so many dimensions that it could be described as a psychological default,' explains Bahns.

'To be clear, we do not mean to suggest that social influence doesn't happen in relationships; however, there's little room for influence to occur when partners are similar at the outset of relationships.'

The pair think that our drive to select like-minded partners could be much stronger than previously assumed.